The invention is directed to a CO or CO.sub.2 waveguide laser of the sealed-off type having transversal excitation and which contains two metal electrodes that serve as waveguide surfaces which are separated from one another by inserter strips. Such a laser is known from IEEE Journal of Quantum Electronics, Vol. QE 20, No. 3, March 1984, incorporated herein by reference. Two aluminum electrodes of polished aluminum are utilized therein. Just like many other metals, however, aluminum reacts with the oxygen of the plasma of a CO or CO.sub.2 waveguide laser. It has been shown that the oxygen content in a sealed-off laser is thereby steadily reduced over a long time, so that the laser characteristics change and the useful life does not correspond to what was anticipated. The formation of aluminum oxide under the conditions of laser discharge thereby does not lead to a dense, insensitive layer of aluminum oxide, or does not lead thereto in an adequately short time.